As exemplified by the spinal fixator made by Spinal Orthopedic Devices, Inc. of the United States and sold under the trademark RAZAIAN, the intervertebral locking device of the prior art, comprised of a structure of steel material, is capable of pulling apart two adjacent vertebrae under treatment so that these two vertebrae are separated from each other by an appropriate interval. Such a prior art locking device as mentioned above is in fact not suitable for use in an orthopedic surgery for treating deformities, diseases, and injuries of the vertebrae in view of the fact that the vertebrae have various curvatures. In addition, the steel structure of the prior art spinal fixator described above is known to have a deflection characteristic, which is five to twenty times greater than those of the vertebrae. As a result, the mechanical stress exerting on the vertebrae under treatment is imparted almost entirely to the steel structure of the spinal fixator, thereby resulting in poor contact between the bone graft and the vertebrae under treatment. The formation of new bone is therefore undermined to an extent that the effect of the treatment is substantially compromised, and that the healed vertebrae are susceptible to a side effect which brings about a shrinkage of the vertebrae in question.